1. Field of the Invention
The present invention relates generally to optical waveguide devices, and more particularly to an optical waveguide device having an optical waveguide of a polymer resin material formed using lamination and photolithography.
2. Description of the Related Art
Branch optical waveguide devices with a branch optical waveguide using a polymer resin material have lower light propagation characteristics, but have the advantages of far better productivity and far lower manufacturing costs than quartz branch optical waveguide devices. Accordingly, branch optical waveguide devices are often used as optical module components.
For convenience of description, a description is given of a process for manufacturing a branch optical waveguide device using a polymer resin material. According to actual manufacturing, multiple branch optical waveguides are formed on a silicon wafer in a matrix manner using lamination and photolithography, and the silicon wafer is scribed into pieces at the end. Here, a description is given in such a way as to form a single branch optical waveguide.
A Y-branch optical waveguide device 1 is manufactured through processes shown in FIGS. 1A through 1G. FIG. 1H shows the completed Y-branch optical waveguide device 1. In FIG. 1H, Z1-Z2, X1-X2, and Y1-Y2 indicate the directions of length, width, and thickness (height), respectively, of the Y-branch optical waveguide device 1. The Y-branch optical waveguide device 1 includes a Y-branch optical waveguide 2 made of a polymer resin material and a silicon substrate 3. The Y-branch optical waveguide 2 is formed on the upper surface of the silicon substrate 3. The Y-branch optical waveguide 2 includes a core 4 made of a polymer resin material such as a fluorinated polyimide resin and lower and upper clad layers 5 and 6 surrounding the core 4. The lower and upper clad layers 5 and 6 are also made of a-fluorinated polyimide resin. The core 4, which has a Y-letter shape, includes an entrance-side core 4a and two branch cores 4b and 4c that branch off therefrom. In FIG. 1H, the core 4 is shown with solid lines for convenience of description.
First, as shown in FIG. 1A, a fluorinated polyimide resin having a refractive index of n1 is applied on the surface of the silicon substrate 3 so that the lower clad layer 5 is formed. Then, as shown in FIG. 1B, a fluorinated polyimide resin having a refractive index of n2 (>n1) is applied on the lower clad layer 5 so that a core layer 10 is formed. Then, as shown in FIG. 1C, for instance, photoresist including silicon is applied on the core layer 10 so that a resist layer 11 is formed. A mask member 20 includes a quartz plate 21 and a Y-shaped mask pattern 22 of a chromium film formed on the lower surface of the quartz plate 21. The mask pattern 22 is shown with solid lines for convenience of description. Next, as shown in FIG. 1D, the mask member 20 is adhered onto the resist layer 11, and is exposed to ultraviolet radiation 25 so as to develop the resist layer 11. As a result, as shown in FIG. 1E, a resist mask 12 for dry etching with a reactive ion etching (RIE) apparatus is formed. Next, RIE dry etching is performed so that the Y-shaped core 4 is formed as shown in FIG. 1F. In the above-described dry etching, which is performed with oxygen pressure inside the chamber of the RIE apparatus being set to, for instance, 0.6 Pa, the core 4 is formed so that both side surfaces (the X1-side and X2-side surfaces) thereof are perpendicular to the upper surface of the silicon substrate 3. Next, as shown in FIG. 1G, the resist mask 12 is removed. Finally, a fluorinated polyimide resin having the refractive index of n1 is applied so that the upper clad layer 6 is formed to cover the core 4. As a result, the Y-branch optical waveguide device 1 shown in FIG. 1H is manufactured. Such an optical waveguide manufacturing method is disclosed in Japanese Laid-Open Patent Application No. 7-92338.
The inventor of the present invention has found that in some of the manufactured Y-branch optical waveguide devices 1, a crack 30 is formed in one or both portions of the lower clad layer 5 which portions extend along the lower edges of the side surfaces of the core 4 so that a cross section of the core 4 is distorted as shown also in FIGS. 2A and 2B. In many cases, the crack 30 was formed in a portion of the lower clad layer 5 which portion extends along part of one of the side surfaces of the core 4 near the branch point of the core 4. FIGS. 2A and 2B are cross-sectional views of the Y-branch optical waveguide device 1 of FIG. 1H taken along the line IIA—IIA and the line IIB—IIB, respectively. In FIG. 2B, optical fibers 100 are indicated by two-dot chain lines for convenience of description.
If the cross section of the core 4 is distorted, light that enters the entrance-side core 4a is prevented from being distributed evenly between the branch cores 4b and 4c, thus causing the problem of uneven branching.
Further, in a Y-branch optical waveguide device module formed by connecting optical fibers to the Y-branch optical waveguide device 1 by adhesion, the end of the core 4 (the branch core 4c) is offset with respect to the center of the end of the corresponding optical fiber 100 as shown in FIG. 2B. This causes a problem in that part of the light propagated through the branch core 4c is prevented from entering the optical fiber 100 at the connection of the Y-branch optical waveguide device 1 and the optical fiber 100, thus causing optical loss.
Further, from the observation of the structure after dry etching with the RIE apparatus shown in FIG. 1F, the inventor of the present invention has found that in some of the structures, the crack 30 is formed in one or both portions of the lower clad layer 5 corresponding to the lower edges of the side surfaces of the core 4. FIG. 3 is a cross-sectional view of the structure of FIG. IF taken along the line III—III.